547
Avionics
By JOHN MARRIOTT
WITH the advent of progressively faster submarines (the average
nuclear submarine can travel at well
over 30kt submerged) it is becoming
increasingly important to use vehicles
to hunt them which themselves are
faster. The obvious answer is the aero
plane, but unfortunately airborne sub
marine detectors are not nearly so
efficient as shipborne sonars. The pur
pose of this discusion is to review the
airborne detectors at present available
and to discuss what improvements, if
any, could be effected in them.
Sonobuoys Fixed-wing aircraft, un
fortunately, cannot tow a sonar
through the water as does a ship; they
therefore have to use sonobuoys. These
devices are dropped from the aircraft
and, on hitting the water, deploy their
sonar equipment. The device is con
nected to a radio transmitter so that
signals on VHF or UHF can be passed
to the aircraft. Sonobuoys can be
either active or passive. In the former
case their sonar transmits and re
ceives, in the latter it merely listens
for submarine noises.
An active sonobuoy provides the
monitoring aircraft with a range and
bearing to the target, while the pas
sive type provides no more than an in
dication that a submarine is present
with, perhaps, a rough direction. How
ever, by dropping a number of buoys
in a pre-determined pattern, it is pos
sible to "contain" the submarine and
for the aircraft to get a good fix.
Passive buoys can also be used in
conjunction with underwater charges
which explode and send sound waves
through the water. These waves are
received by the buoys and are also
reflected by any submerged object.
The buoys measure the time difference
between the receipt of the initial sound
wave and its reflection, and thus an
indication of range is obtained.
On board the aircraft the informa
tion obtained from sonobuoys is evalu
ated by a computer and a fix of the
submarine's position displayed.
Sonobuoys can be very effective
against a slow, conventional sub
marine, but a nuclear vessel does not
find it too difficult to avoid detection.
Helicopters Helicopters, because
of their hover capability, can lower a
transmitting sonar transducer into the
water. In the British Type 195 system,
temperature and pressure sensors are
incorporated in the submersive unit.
These feed into a bathythermograph
recorder in the aircraft which pro
duces a pen trace showing the tem
perature of the water against depth.
The operator can thus see for himself
the depths of the temperature layers.
These tend to reflect or refract sonar
waves and submarines often try to
hide beneath them. With the helicop
ter sonar, however, this becomes much
more difficult as the operator merely
has to lower the sonar transducer
below the level of the layers.
Helicopter sonar has proved ex
tremely useful, but it still cannot be
towed through the water; thus, if a
helicopter detects a submarine moving
at high speed, it has to raise its sonar
transducer, proceed ahead of the
estimated position of the submarine
and lower it again, hoping to detect it
once more: a process known as dunk
ing or dipping.
Other devices There are two other
methods of detecting a submerged
submarine from the air. The first,
called Autolycus in Britain and Sniffer
in America, is a device which detects
the fumes emitted by a diesel-powered
submarine into the atmosphere. It is
not a primary means of detection and
at best it can only tell the aircraft that
a diesel submarine was recently pre
sent (and has now dived), or is
present but for some reason the snor
kel has not been detected. As more
and more submarines become nuclear
powered the device will lose its
already limited usefulness.
A somewhat better device is Mad or
magnetic anomaly detector. This is a
magnetometer installed on the end of
a long boom projecting from the tail
of the aircarft. It indicates on a meter
in the aircraft any disturbance in the
Earth's magnetic field caused by the
presence of a large metal mass, such
as a submarine. The long boom is
necessary to keep the device clear of
the magnetic field of the aircraft. It is
essentially a low-level device and its
range of detection is very small, so
again it is not a primary detector.
Radar Although nuclear submarines
can stay completely submerged for ex
tended periods, the diesel boat has
either to surface or expose a snorkel
whenever it needs to charge its bat
teries. In addition submarines about to
attack often have to surface in order
to detect their prey in bad weather,
and at other times they probably have
to expose a radar mast or a periscope.
Thus there is still a chance of detect
ing a submarine either fully surfaced
or exposing some part of itself. Here
radar comes into its own.
Most fixed-wing maritime aircraft
are fitted with a scanning radar, use
ful both for picking up surface ships
and submarines, or snorkels, radars
and periscopes. The range of detec
tion of this latter group is not large
and in rough seas it is often difficult
to pick them out of the sea clutter.
For all that, radar is a very effective
tool. It has its limitations in that a
submarine can receive a radar trans
mission from an aircraft well outside
the detection range of that aircraft,
and can dive deep long before it
arrives. However, it also encourages
submarines to remain submerged, or
at least keep their radars and snorkels
down for longer periods than their
crews might wish.
Conversely, maritime aircraft are
fitted with receivers which can detect
submarines' radar transmissions be
fore being itself detected.
While the main purpose of mari
time aircraft is, of course, to attack
submarines, it is also very important
that they should report their positions
and routes, so that convoys and ship
ping generally can be warned and
ships diverted to attack. It follows
that not only must the aircraft deter^
mine accurately where the submarine
is in relation to itself, but must also
know its own geographical position
with considerable accuracy. Such air
craft are therefore fitted with a com
plete operations room in which the
detections made by the various sensors
can be evaluated and their positions
displayed. In addition they must have
the most accurate navigational sys
tems possible.
In the Nimrod, for example, all
sensors feed into a tactical display.
A computer processes all detections,
filters them and displays on a cathode-
ray tube those which are required.
The computer also predicts the tar
get's future position, track and speed
and will even indicate the intercept
point and weapon splash and release
points. The whole operation is con
trolled by a single crew member,
known as the tactical navigator.
Seated alongside him is the routine
navigator, who is responsible for the
long-term navigation of the aircraft.
The primary navigational device is an
inertial system, but Doppler is also
fitted, and the combination of the two
generates extremely accurate naviga
tion data.
New sensors Since none of the ex
isting underwater detectors can be
regarded as entirely satisfactory, the
hunt for new methods goes on. One
possibility is the use of infra-red
techniques. A submarine will have a
different temperature to the surround
ing water and a nuclear submarine,
particularly, will heat the water
around it due to its nuclear discharge.
Aircraft fitted with IR detectors
could note the thermal variations and
thus determine the position of the
submarine. The problem is that these
temperature differences are very
small, sometimes only 0-001°C. Pre
sent IR sensors fitted in aircraft can
detect differences of only about 0-3°C,
but techniques are being con
tinually improved and it is possible
that this method will one day become
really practicable.
Another possible detection tool is
the laser. By using very short pulses
it is possible to detect the reflections
of a laser beam from a submarine and
discriminate them from reflections
from the water surface. Lasers, how
ever, are essentially directional de
vices and would not be of much value
for overall surveillance.